Method and apparatus for optical asynchronouos sampling signal measurements

ABSTRACT

A method and a system for measuring an optical asynchronous sample signal. The system for measuring an optical asynchronous sampling signal comprises a pulsed optical source capable of emitting two optical pulse sequences with different repetition frequencies, a signal optical path, a reference optical path, and a detection device. Since the optical asynchronous sampling signal can be measured by merely using one pulsed optical source, the complexity and cost of the system are reduced. A multi-frequency optical comb system using the pulsed optical source and a method for implementing the multi-frequency optical comb are further disclosed.

PRIORITY CLAIM

This application claims priority to PCT Patent Application No.PCT/CN2013/072093 published as “A method and apparatus for opticalasynchronous sampling signal measurement” by Zheng Zheng et al., filedMar. 1, 2013, which claims priority to Chinese application No.201210052940.6 filed Mar. 2, 2012, Chinese application No.201210052680.2 filed Mar. 2, 2012, Chinese application No.2012100137481.1 filed May 4, 2012, Chinese application No.2012100137119.4 filed May 4, 2012, and Chinese application No.201210062796.9 filed Feb. 28, 2013 the specification and drawings ofeach of which are herein expressly incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The invention relates to the field of optical measurement, andparticularly relates to a method and apparatus for optical asynchronoussampling signal measurement.

BACKGROUND OF THE INVENTION

Optical asynchronous sampling is a technique in which two precisefrequency locked optical frequency combs with a small frequency spacingdifferences are used to achieve high-precision time domain “equivalentsampling” signal measurement. The high-precision time domain “equivalentsampling” signal measurement is similar to the principle used in theequivalent sampling oscilloscope.

SUMMARY OF THE INVENTION

The optical asynchronous sampling technique has been applied topump-probe, terahertz time-domain spectroscopy, distance measurement andother fields. However, the light sources used previously by researchersare two individual lasers with a specified repetition frequencydifference. The two lasers require a complex electronic feedback controlsystem to keep the frequency difference stable and maintain phaselocking, which is complex, high cost and hard to use. One light sourcecould emit two laser pulses with different repetition rates byleveraging the modal dispersion, the polarization mode dispersion, thebirefringence and the chromatic dispersion in the optical resonantcavity. Due to the stability of such kinds of dispersion, the repetitionfrequency difference of the two laser pulses could be quite stable.Dual-repetition-frequency (dual-frequency) light source realized byusing this method has the advantages of structure simplicity, easy tointegrate etc., so that the optical asynchronous sampling signalmeasurement system is simpler and more convenient to implement.

DETAILS OF THE INVENTION

In view of the problems existing in the prior art, the inventionprovides methods and apparatuses for optical asynchronous samplingsignal measurement.

The application provides a method for optical asynchronous samplingsignal measurement, whose characteristics comprise:

Step 1, a pulsed light source emits more than two optical pulsesequences with different repetition frequencies, one of which with thefirst repetition frequency of f₁ is the first optical pulse sequence,the other with the second repetition frequency of f₂ is the secondoptical pulse sequence, where Δf is the repetition frequency differencebetween the first optical pulse sequence and the second optical pulsesequence, i.e. Δf=|f₁−f₂|;

Step 2, the first optical pulse sequence is transformed into a signalpulse sequence through the signal optical path, the second optical pulsesequence is transformed into a reference pulse sequence through thereference optical path;

Step 3, the signal pulse sequences and the reference pulse sequenceinteract in the detection device to obtain the asynchronous samplingsignal determined by f₁ and f₂; and

Step 4, the time axis of the asynchronous sampling signal is transformedbased on the transform formula ΔT=ΔτΔf/f₁, where Δτ is the temporalposition in asynchronous sampling signal, ΔT is the actual temporalposition, to retrieve the time domain information. The time domainspectral information can be obtained by means of transforming betweentime domain and frequency domain.

The measurement method above can be applied in the following fiveexamples, respectively, for the terahertz signal measurement, thepumped-probe signal measurement, the time-domain spectroscopymeasurement, the optical distance measurement based on correlationsignal measurement and the spectrum measurement based on non-correlationsignal measurement:

1. In an example, in step 2, the first optical pulse sequence is inputto a terahertz emission device, after going through the power control,the pulse waveform conversion, the polarization control and/or thefrequency doubling transformation, the signal pulse sequence is formedwhen the terahertz pulse sequences generated by terahertz emissiondevice is passed through the device under test. The second optical pulsesequence is transformed into the reference pulse sequence, after goingthrough the power control, pulse waveform conversion, polarizationcontrol and/or frequency doubling transformation; in step 3, the signalpulse sequences and the reference pulse sequences are input to aterahertz receiving device together, after detected by a photodetector,the asynchronous sampling signal is obtained; in step 4, afterdata-processing of the asynchronous sampling signal, the terahertz timedomain information and/or the time-domain spectroscopy information canbe obtained.

Specifically, the aforementioned asynchronous sampling signalmeasurement method for the terahertz signal measurement includes:

The first step, the pulsed light source generates optical pulsesequences with more than two different repetition frequencies, which aresplit into the first optical pulse sequence with the repetitionfrequency of f₁ and the second optical pulse sequence with therepetition frequency of f₂ after the pulse beam splitting, therepetition frequency difference (Δf) between the first optical pulsesequence and the second optical pulse sequence is equal to the absolutevalue of f₁ minus f₂ (|f₁−f₂|).

The second step, the first optical pulse sequence or the part of it ispassed through the power amplification, the power control, the pulsewaveform conversion, the polarization control, the frequency doublingtransformation, to form a pump optical pulse sequence, which is inputinto the terahertz emission device, through the photoconductive method,the optical rectification method or the surface effect method togenerate a terahertz signal. The terahertz signal is formed into thesignal pulse sequence after passing through the device under test; thesecond optical pulse sequence or the part of it is passed through theoptical power amplification, the power control, the pulse waveformconversion, the polarization control, the frequency doublingtransformation, to form an optical pulse sequence, namely the referencepulse sequence;

The third step, the signal pulse sequence and the reference pulsesequence are input into the terahertz receiver together, based on thephotoconductive sampling or electro-optic sampling method, anasynchronous sampling signal related to the terahertz time domainwaveform is obtained;

The fourth step, by means of the data processing of the asynchronoussampling signal, the time axis conversion relationship between theterahertz time-domain signal and the asynchronous sampling signal isΔT=ΔτΔf/f₁, where Δτ is the temporal position of the asynchronoussampling signal, ΔT is the temporal position of the terahertztime-domain signal. Furthermore, through the transformation between thetime domain and the frequency domain, the terahertz time-domainspectroscopy information can be obtained.

2. In an example, in step 2, the first optical pulse sequence istransformed into a signal pulse sequence after going through the powercontrol, the pulse waveform conversion, the polarization control and/orthe frequency doubling transformation. The second optical pulse sequenceis transformed into a reference pulse sequence after going through thepower control, the pulse waveform conversion, the polarization controland/or the frequency doubling transformation; in step 3, the signalpulse sequence and the reference pulse sequence are input to the deviceunder test, and then the signal pulse sequence is detected by thephotodetector to get an asynchronous sampling signal; in step 4, afterthe data processing of the asynchronous sampling signal, the pump-probesignal of the device under test can be obtained.

Specifically, the asynchronous sampling signal measurement methoddescribed above for the pump probe signal measurement includes:

The first step, the pulsed light source generates optical pulsesequences with more than two different repetition frequency, which aresplit into the first optical pulse sequence with the repetitionfrequency of f₁ and the second optical pulse sequence with therepetition frequency of f₂ after the pulse beam splitting, therepetition frequency difference between the first optical pulse sequenceand the second optical pulse sequence is Δf=|f₁−f₂|.

The second step, the first optical pulse sequence or the part of it ispassed through the power amplification, the power control, the pulsewaveform conversion, the polarization control, the beam splitter or thefrequency doubling transformation, to form a signal pulse sequence; thesecond optical pulse sequence or a part of it is passed through thepower amplification, the power control, the pulse waveform conversion,the polarization control, the beam splitter or the frequency doublingtransformation, to form a reference pulse sequence;

The third step, the signal pulse sequence and the reference pulsesequence are incident on the device under test in a collinear ornon-collinear configuration, by measuring the signal pulse sequence witha photodetector, an asynchronous sampling signal related to the pumpprobe signal is obtained;

The fourth step, by means of the data processing of the asynchronoussampling signal, the time axis conversion relationship between theterahertz time-domain signal and the asynchronous sampling signal isΔT=ΔτΔf/f₁, where Δτ is the temporal position of the asynchronoussampling signal, and ΔT is the temporal position of the pump probesignal.

3. In an example, in step 2, the first optical pulse sequence is inputto the device under test after going through the power amplification,the pulse waveform conversion, the polarization control and the spectralshift, to form a signal pulse sequence; while the second optical pulsesequence is transformed into a reference pulse sequence through thepower amplification, the pulse waveform conversion, the polarizationcontrol; in step 3, the time domain correlation signal is generatedbetween the reference pulse sequence and the signal pulse sequence; instep 4, after the data processing of the signal, the time domainspectroscopy information of the device under test can be obtained.

Specifically, the asynchronous sampling signal measurement methoddescribed above for the time domain spectroscopy measurement includes:

The first step, the pulsed light source generates optical pulsesequences with more than two different repetition frequencies, which canbe split into the first optical pulse sequence with the repetitionfrequency of f₁ and the second optical pulse sequence with therepetition frequency of f₂ after the pulse beam splitting, therepetition frequency difference between the first optical pulse sequenceand the second optical pulse sequence is Δf=|f₁−f₂|.

The second step, the first optical pulse sequence or a part of it ispassed through the power amplification, the power control, the pulsewaveform conversion, the polarization control, resulting in the spectraltransformation, and is further passed through the device under test toform a signal pulse sequence; the second optical pulse sequence or apart of it is passed through the power amplification, the power control,the pulse waveform conversion, the polarization control, the spectraltransformation, to form a reference pulse sequence;

The third step, the signal pulse sequence and the reference pulsesequence are incident upon the photodetector, the signal pulse sequenceis detected by a photodetector to get the asynchronous sampling signal.

The fourth step, by means of the data processing of the asynchronoussampling signal, the time axis conversion relationship between theterahertz time-domain signal and the asynchronous sampling signal isΔT=ΔτΔf/f₁, where Δτ is the temporal position of the asynchronoussampling signal, ΔT is the temporal position of the time domain signal,furthermore, through the signal transformation between the time domainand the frequency domain, the time-domain spectroscopy information canbe obtained.

Specifically, the reason that the first optical pulse sequence describedabove can be spectrally shifted to have an overlapped spectral rangewith the reference pulse sequence.

4. In an example, in step 2, the first optical pulse sequence can besplit into two branches after going through the power control, the pulsewaveform conversion, the polarization control and/or the wavelengthshift, one of them can be passed through the calibration optical path togenerate a calibration pulse sequence, the other can be passed throughthe target optical path to generate a target pulse sequence. Thecalibration pulse sequence and the target pulse sequence merge into thesignal pulse sequence. The second optical pulse sequence, after goingthrough the power control, the pulse waveform conversion, thepolarization control and/or the wavelength shift to form a referencepulse sequence; in step 3, a field correlation signal or an intensitycorrelation signal between the reference pulse sequence and the signalpulse sequence can be obtained; in step 4, according to the aboveobtained signal, the time difference between the target pulse and theclosest calibration pulse before it in the signal pulse sequence can becalculated to get the optical distance difference between the targetoptical path and the calibration optical path.

Specifically, the asynchronous sampling signal measurement methoddescribed above for the optical path measurement includes:

The first step, the pulsed light source generates optical pulsesequences with more than two different repetition frequency, which canbe split into the first optical pulse sequence with the repetitionfrequency of f₁ and the second optical pulse sequence with therepetition frequency of f₂ after the pulse beam splitting, where therepetition frequency difference between the first optical pulse sequenceand the second optical pulse sequence is Δf=|f₁−f₂|.

In the second step, the first optical pulse sequence can be passedthrough the calibration optical path to generate a calibration pulsesequence, the first optical pulse sequence is then passed through thetarget optical path to generate a target pulse sequence, and thecalibration pulse sequence and the target pulse sequence merge into thesignal pulse sequence;

The third step, measures the field correlation signal or the intensitycorrelation signal generated between the signal pulse sequence and thereference pulse sequence;

The fourth step, according to the time domain correlation signal, thetime difference between the target pulse and the closest calibrationpulse before it in the signal pulse sequence can be calculated to getthe optical distance difference between the target optical path and thecalibration optical path.

Hereto, when the time domain correlation signal is the field correlationsignal between the reference optical pulse sequence and the signaloptical pulse sequence, the first optical pulse sequence and/or thesecond optical pulse sequence go through the wavelength shift to makethe spectra of the reference pulse sequence and the signal pulsesequence have a good overlap.

Hereto, the first optical pulse sequence can be incident on acalibration surface to generate the calibration pulse sequence by thereflection from the calibration surface; and the first optical pulsesequence can be incident on a target surface to generate the targetpulse sequence by the reflection from the target surface.

Hereto, the first optical pulse sequence could be passed through acalibration delay to generate the calibration pulse sequence, and thefirst optical pulse sequence could be passed through a target delay togenerate the target pulse sequence.

In the fourth step, calculate the optical distance according to thefollowing formula: d=v_(g)(ΔτΔf/f_(p)+n/f_(p)), where d is the opticaldistance difference between the target optical path and the calibrationoptical path, v_(g) is the group velocity of the signal optical pulse,Δf is the repetition frequency difference between the first opticalpulse sequence and the second optical pulse sequence, f₁ is therepetition frequency of the first optical pulse sequence, Δτ is theactual measured time difference between the target pulse and the closestcalibration pulse before the target pulse in the time domain correlationsignal, n is an integer, and nv_(g)/f_(p) is the ambiguity range.

5. In an example, in step 2, the first optical pulse sequence can besplit into two branches after going through the power controller, thepulse waveform conversion and the polarization controller. One of thebranches can be passed through the calibration optical path to generatea calibration pulse sequence, the other branch can be passed through thetarget optical path to generate a target pulse sequence, and then thecalibration pulse sequence and the target pulse sequence can be mergedinto a signal pulse sequence. The second optical pulse sequence can bepassed through the power control, the pulse waveform conversion and thepolarization control to form a reference pulse sequence; in step 3, thesignal pulse sequence and the reference pulse sequence are input into apulse interaction device, in which the characteristics of the pulse inthe signal pulse sequence that overlaps with a pulse of the referencepulse sequence in the time domain experiences a change, and then thesignal pulse sequence can be measured to give an asynchronous samplingsignal; in step 4, according to the temporal position of the pulse inthe asynchronous sampling signal, whose characteristics are changed, thetime difference between the target pulse and the closest calibrationpulse in front of the target pulse in the signal pulse sequence can becalculated to get the optical distance difference between the targetoptical path and the calibration optical path.

Specifically, the asynchronous sampling signal measurement methoddescribed above for the optical path measurement includes:

The first step, the pulsed light source generates optical pulsesequences with more than two different repetition frequencies, which aresplit into the first optical pulse sequence with the repetitionfrequency of f₁ and the second optical pulse sequence with therepetition frequency of f₂ after the pulse beam splitting. Therepetition frequency difference between the first optical pulse sequenceand the second optical pulse sequence is Δf=|f₁−f₂|.

In the second step, the first optical pulse sequence can be split intotwo branches and passed through the power controller, the pulse waveformconversion, and the polarization controller. One of the branches can bepassed through the calibration optical path to generate a calibrationpulse sequence, the other branch can be passed through the targetoptical path to generate a target pulse sequence. The calibration pulsesequence and the target pulse sequence merge into a signal pulsesequence. The second optical pulse sequence can be passed through thepower control, the pulse waveform conversion and the polarizationcontrol to form a reference pulse sequence;

In the third step, the signal pulse sequence and the reference pulsesequence are sent into the pulse interaction device, in which thecharacteristics of the pulse from the signal pulse sequence which can beoverlapped in the time domain with a pulse from the reference pulsesequence experience a change, and then measuring the signal pulsesequence to provide the asynchronous sampling signal;

The fourth step, according to the asynchronous sampling signal, the timedifference between the target pulse and the closest calibration pulsebefore the target pulse in the signal pulse sequence can be calculatedto give the optical distance difference between the target optical pathand the calibration optical path.

In the fourth step, the optical path can be calculated according to thefollowing formula:

d=v _(g)(ΔτΔf/f _(p) +n/f _(p)),

where d is the optical distance difference between the target opticalpath and the calibration optical path, v_(g) is the group velocity ofthe signal optical pulse, Δf is the repetition frequency differencebetween the first optical pulse sequence and the second optical pulsesequence, f₁ is the repetition frequency of the first optical pulsesequence, Δτ is the actual measured time difference between the targetpulse and the closest calibration pulse before the target pulse in thetime domain correlation signal, n is an integer, and nv_(g)/f_(p) is theambiguity range.

The application provides an apparatus for optical asynchronous samplingsignal measurement, whose characteristics comprise:

The pulsed light source generates optical pulse sequences with differentrepetition frequencies, one with the repetition frequency of f₁ iscalled the first optical pulse sequence, one with the repetitionfrequency of f₂ is called the second optical pulse sequence;

The signal light path can be used to transform the first optical pulsesequence into a signal pulse sequence;

The reference light path can be used to transform the second opticalpulse sequence into a reference pulse sequence;

The detection device can be used to realize the interaction between thesignal pulse sequence and the reference pulse sequence and obtain anasynchronous sampling signal.

In an example, the pulsed light source contains only one resonantcavity, by means of the modal dispersion, the polarization modedispersion, the birefringence or the chromatic dispersion in the cavity,and it can realize simultaneously emitting optical pulse sequences withtwo different repetition frequencies.

The measuring apparatus can be applied in the following five examples,respectively for terahertz signal measurement, pump-probe signalmeasurement, time-domain spectroscopy measurement, optical distancemeasurement based on correlation signal measurement, and opticaldistance measurement based on non-correlation signal measurement:

1. In an example, the signal light path includes an optical poweramplifier, a dispersion control device, a polarization control device, afrequency doubling crystal, a terahertz emission device and a deviceunder test; where the reference light path includes an optical poweramplifier, a dispersion control device, a polarization controller and afrequency doubling crystal; and the detection device comprises aterahertz receiver composed of an electro-optic device and aphotodetector, or composed of a photoconductive switch.

Specifically, the asynchronous sampling signal measurement apparatusdescribed above for the terahertz signal measurement includes:

The pulsed light source outputs optical pulse sequences with more thantwo different repetition frequencies;

The pulse splitting and processing device, including an optical fibercoupler, a beam splitter prism, a beam splitter, the optical filter, aband-pass filter or a wavelength division multiplexer, can be used todivide the first optical pulse sequence and the second optical pulsesequence generated by the pulsed light source into two branches;

The signal light path, including an optical power amplifier, adispersion control device, a polarization control device, a frequencydoubling crystal, converts the first optical pulse sequence into a pumpoptical pulse sequence. The signal light path also includes theterahertz emission device, such as an electro-optic device, aphotoconductive switch device or a surface effect device that radiatesterahertz wave. The signal light path also can include the device undertest;

The reference light path, including an optical power amplifier, adispersion control device, a polarization control device, a frequencydoubling crystal, converts the second optical pulse sequence into areference pulse sequence;

The detection device, including a terahertz receiver composed of anelectro-optic device and a photodetector, or composed of aphotoconductive switch, wherein the electro-optic device includes InAs,GaAs, InSb, ZnTe, LiTaO₃, DAST, electro-optic polymer materials andetc., the photodetector can be a PIN detector, an APD detector, aphotomultiplier tube or a balanced detector.

In the optical asynchronous sampling signal measurement apparatus forthe terahertz signal measurement, the wavelengths of the pump opticalpulse sequence and the reference optical pulse sequence can be the sameor different. The wavelengths of the pump optical pulse sequence and thereference optical pulse sequence can be the same as the wavelength of apulse sequence in the output signal of the pulsed light source, or canbe converted from the wavelength of a pulse sequence of the pulsed lightsource output to another wavelength, but the repetition frequency of thepump optical pulse sequence and that of the reference optical pulsesequence must be different, where the repetition frequencies are aninteger multiple times of each other.

2. In one example, the signal light path includes an optical poweramplifier, a dispersion control device, a polarization control deviceand/or a frequency doubling crystal; the reference light path includesan optical power amplifier, a dispersion control device, a polarizationcontroller and/or a frequency doubling crystal; the detection devicecomprises a device under test, a filtering device and a photodetector.

Specifically, the asynchronous sampling signal measurement apparatusdescribed above for the pump detection signal measurement includes:

The pulsed light source emits optical pulse sequences with more than twodifferent repetition frequencies, where the different repetitionfrequencies are not an integer multiple of each other.

The signal light path, including an optical power amplifier, adispersion control device, a polarization control device, a frequencydoubling crystal, converts the first optical pulse sequence into asignal pulse sequence.

The reference light path, including an optical power amplifier, adispersion control device, a polarization control device, a nonlinearoptical device, converts the second optical pulse sequence into areference pulse sequence;

The detection device, including a device under test, a filtering deviceand a photodetector, wherein the filtering device can be the deviceswith the filtering effect like an optical filter, a band-pass filter ora wavelength division multiplexer, a polarizing beam splitter cube or apolarizer with the analyzer function; wherein the photodetector includesa PIN detector, an APD detector, a photo multiplier tube or a balanceddetector.

3. In one example, the signal light path includes an optical poweramplifier, a dispersion control device, a polarization control deviceand a nonlinear optical device; the reference light path includes anoptical power amplifier, a dispersion control device, a polarizationcontroller; and the detection device comprises a device under test, afilter and a photodetector.

Specifically, the asynchronous sampling signal measurement apparatus forthe time domain spectroscopy signal measurement described aboveincludes:

The pulsed light source outputs optical pulse sequences with more thantwo different repetition frequencies.

The signal light path, including an optical power amplifier, adispersion control device, a polarization controller and a nonlinearoptical device, converts the first optical pulse sequence into a signalpulse sequence; wherein the function of the nonlinear optical device isto transform the spectrum of the signal pulse sequence so that it has anoverlap with the spectrum of the reference pulse sequence;

The reference light path, including an optical power amplifier, adispersion control device, a polarization control device, converts thesecond optical pulse sequence into a reference pulse sequence;

The detection device includes a device under test, a filtering deviceand a photodetector.

4. In one example, the signal light path includes an optical poweramplifier, a dispersion control device, a polarization control device, anonlinear optical device, a calibration path and a target path; thereference light path includes an optical power amplifier, a dispersioncontrol device, a polarization controller and/or a nonlinear opticaldevice; where the detection device includes a frequency doublingcrystal, a filtering device and a photodetector.

Specifically, the optical asynchronous sampling signal measurementapparatus described above for the optical distance measurement,depending upon whether the measured signal is a field correlation signalor an intensity correlation signal, in the following two different butsimilar configurations of the apparatuses are feasible and could bepresented in the following:

The first apparatus can be based to measure the optical distance bymeasuring the field correlation signal, which needs the signal pulsesequence and the reference pulse sequence to be spectrally overlapped,including:

The pulsed light source outputs an optical pulse sequence with more thantwo different repetition frequencies;

The signal light path includes an optical power amplifier, a dispersioncontrol device, a polarization control device, a nonlinear opticaldevice, a calibration path and a target path, wherein the nonlinearoptical device can broaden or shift the spectra of the first opticalpulse sequence, and some new spectral components can be created, so thatthe spectrum of the signal pulse sequence and the spectrum of thereference pulse sequence overlap;

The reference light path includes an optical power amplifier, adispersion control device, a polarization control device, a nonlinearoptical device, wherein the nonlinear optical device can broaden orshift the spectrum of the second optical pulse sequence, and generatesome new spectral components, so that the spectrum of the referencepulse sequence and the spectrum of the signal pulse sequence overlap;

The detection device includes a filtering device and a photodetector.

The second apparatus can be used to measure the intensity correlationsignal by measuring the optical path, which does not need the spectrumof the signal pulse sequence and the spectrum of the reference pulsesequence to overlap. The second apparatus includes:

The pulsed light source outputs an optical pulse sequence with more thantwo different repetition frequencies.

The signal light path includes an optical power amplifier, a dispersioncontrol device, a polarization control device, a calibration path and atarget path;

The reference light path includes an optical power amplifier, adispersion control device, a polarization control device;

The detection device includes a frequency doubling crystal with a PINdetector, an APD detector or a frequency doubling detector composed ofthe photomultiplier tube or a two-photon-absorption photodetectiondevice.

5. In an example, characterized in that, the signal light path includesan optical power amplifier, a dispersion control device, a polarizationcontrol device, a calibration path and a target path; the referencelight path includes an optical power amplifier, a dispersion controldevice, a polarization control device; where the detection deviceincludes a pulse interaction device, a filtering device and aphotodetector.

Specifically, the above can be the optical asynchronous sampling, signalmeasurement system, whose goal is to measure the optical distance. Theapparatus described above can be used for the optical path measurement.The measured asynchronous sampling signal is no longer the correlationsignal between the reference pulse sequence and the signal pulsesequence. The apparatus includes:

The pulsed light source outputs optical pulse sequences with more thantwo different repetition frequencies;

The signal light path includes an optical power amplifier, a dispersioncontrol device, a polarization control device, a nonlinear opticaldevice, a calibration path and a target path;

The reference light path includes an optical power amplifier, adispersion control device, and a polarization control device.

The detection device includes a pulse interaction device, a filteringdevice and a photodetector.

In one example, characterized in that, the pulse device includes asemiconductor optical amplifier, a saturable absorber, an all-opticalswitch and an all-optical logic gate.

The application provides a method for generating a multi frequencyoptical comb, including:

Step 1, the pulsed laser outputs two or more than two optical pulsesequences with different center wavelengths and different repetitionrates, where the maximum value of the full width at half maximum of theadjacent optical pulse sequences with different center wavelengths inthe spectrum is less than the difference of the two center wavelengths;

Step 2, the optical pulse sequence with different center wavelengths anddifferent repetition rates emitted by the pulsed laser can be passedthrough the nonlinear optical process to make the spectra of the opticalpulse sequences with one or more different center wavelengths broadenand overlap, so that it has an optical comb with more than two kinds ofdifferent repetition frequencies in the wavelength region, where thespectra overlap.

In an example, in step 2, the optical pulse sequence with differentcenter wavelengths and different repetition rates can be broadenedtogether through an element, which can achieve a nonlinear opticalprocess, to broaden the spectrum, so that the spectrum of the firstoptical pulse sequence and the second optical pulse sequence overlapafter broadening.

In an example, the step 2 can be further divided into:

Step 21, the optical pulse sequence output by the pulsed laser can besplit into the first optical pulse sequence and the second optical pulsesequence by the optical splitting device, the center wavelength of thefirst optical pulse sequence is the first wavelength, the repetitionfrequency of the first optical pulse sequence is the first frequency,the center wavelength of second optical pulse sequence is the secondwavelength, the repetition frequency of the second optical pulsesequence is the second frequency;

Step 22, the first optical pulse sequence and/or the second opticalpulse sequence can be broadened respectively after going through thenonlinear optical process to make the spectrum broader, so that thespectrum of the first optical pulse sequence and that of the secondoptical pulse sequence overlap after broadening.

In one example, the nonlinear optical process is based on the four-wavemixing, the self-phase modulation, the cross-phase modulation, thestimulated Raman scattering effect or their combinations thereof.

The application provides a multi-frequency optical comb apparatus,including:

A pulsed laser output with more than two kinds of optical pulsesequences each with different central wavelengths, where the maximumvalue of the full width at half maximum of the optical pulse sequenceswith different center wavelengths is less than the difference of thecenter wavelengths, and the average group velocity in the pulsed lasercavity is not the same for the different center wavelengths, so that therepetition frequency of the optical pulse sequences with differentcenter wavelengths is different;

The optical pulse sequence emitted by the pulsed laser can be passedthrough the nonlinear optical apparatus to broaden the spectrum of theoptical pulse sequence with one or more different center wavelengths, sothat the spectra of the optical pulse sequences with different centerwavelengths overlap after broadening.

In an example, the nonlinear optical apparatus includes:

The optical splitter divides the optical pulse sequence output by thepulsed laser into many optical pulse sequences, the center wavelengthsof each of which are different, and the value of the full width at halfmaximum of the spectra is less than the difference of the adjacentcenter wavelengths;

The nonlinear optical device broadens the spectra of the optical pulsesequences with one or more different center wavelengths, so that thespectra of the optical pulse sequences with different center wavelengthscould overlap after broadening.

In an example, the shape of the pulsed laser cavity can be a linearcavity, a folded cavity, a ring cavity or an “8” shape cavity.

In an example, the pulsed laser can be an active mode-locked laser, apassive mode-locked laser or a mixed mode locked laser.

In an example, the nonlinear optical device can be single-mode opticalfiber, a high nonlinear optical fiber, Optical fiber, a photonic crystalfiber or a nonlinear integrated optical waveguide.

In an example, the optical splitting device can be an optical fibercoupler, a beam splitter prism, a beam splitter, an optical filter, aband-pass filter or a wavelength division multiplexer.

DESCRIPTION OF THE FIGURES

Below with reference to the figures, the application will be describedin further detail, in which:

FIG. 1 is a schematic diagram showing an optical asynchronous samplingsignal measurement apparatus;

FIG. 2 is a schematic diagram showing a first optical asynchronoussampling signal measurement apparatus for terahertz signal measurement;

FIG. 3 is a schematic diagram showing a second optical asynchronoussampling signal measurement apparatus for terahertz signal measurement;

FIG. 4 is a schematic diagram showing a third optical asynchronoussampling signal measurement apparatus for terahertz signal measurement;

FIG. 5 is a schematic diagram showing a first optical asynchronoussampling signal measurement apparatus for pump-probe signal measurement;

FIG. 6 is a schematic diagram showing a second optical asynchronoussampling signal measurement apparatus for pump-probe signal measurement;

FIG. 7 is a schematic diagram showing a third optical asynchronoussampling signal measurement apparatus for pump-probe signal measurement;

FIG. 8 is a schematic diagram showing a fourth optical asynchronoussampling signal measurement apparatus for pump-probe signal measurement;

FIG. 9 is a schematic diagram showing an optical asynchronous samplingsignal measurement apparatus for time domain spectroscopy measurement;

FIG. 10 is a schematic diagram showing a dual-wavelength pulse laserapparatus;

FIG. 11 is the output optical spectrum of the dual-wavelength pulselaser.

FIG. 12 is the radio-frequency (RF) spectrum of the output of thedual-wavelength pulse laser after photodetection;

FIG. 13 is a schematic diagram showing an optical asynchronous samplingsignal measurement apparatus for optical distance measurement;

FIG. 14 is the spectrum of the optical pulse with the center wavelengthof 1532 nm, obtained by filtering the output of the dual-wavelengthpulse laser;

FIG. 15 is the spectrum of the optical pulse with the center wavelengthof 1547 nm, obtained by filtering the output of the dual-wavelengthpulse laser;

FIG. 16 is the spectrum of the optical pulse with the center wavelengthof 1547 nm after been amplified by the optical amplifier 2 and itsspectrum broadened;

FIG. 17 is the spectrum of the output light passed through the band-passfilter with a passband of 1528 nm to 1536 nm after the spectrumbroadening;

FIG. 18 is the field correlation signal measured by the oscilloscope;

FIG. 19 is the schematic of the intensity correlation signal;

FIG. 20 is a schematic diagram of one optical asynchronous samplingsignal measurement apparatus which can be used for light pathmeasurement;

FIG. 21 is the output optical spectrum of a dual-wavelength pulse laser;

FIG. 22 is the radio-frequency spectrum of the output of adual-wavelength pulse laser;

FIG. 23 is the time domain graph measured by an oscilloscope.

FIG. 24 is a schematic diagram of another optical asynchronous samplingsignal measurement apparatus which can be used for optical distancemeasurement;

FIG. 25 is a schematic diagram of a dual-wavelength pulse laserapparatus;

FIG. 26 is a schematic diagram of the apparatus using a dual-wavelengthpulse laser to realize a multi-frequency optical comb;

FIG. 27 is the output optical spectrum of a dual-wavelength mode-lockedlaser;

FIG. 28 is the radio-frequency spectrum of the signal after thephotodetection of the output of the dual-wavelength mode-locked laser;

FIG. 29 is the spectrum of the optical pulse with the center wavelengthof 1535 nm, obtained by filtering the output of the dual-wavelengthmode-locked laser;

FIG. 30 is the spectrum of the optical pulse with the center wavelengthof 1557 nm, obtained by filtering the output of the dual-wavelengthpulse laser;

FIG. 31 is the spectrum of the optical pulse with the original centerwavelength of 1557 nm, after been amplified by the optical amplifier andits spectrum broadened by transmitting through the single mode opticalfiber;

FIG. 32 is the spectrum of the output light through the band-pass filterwith a passband of 1528 nm to 1536 nm after the spectrum of the pulsewhose original center wavelength is 1557 nm is broadened;

FIG. 33 is the spectrum of the output optical pulses of thedual-wavelength pulse laser, after the power amplification and thespectrum broadening realized by the optical amplifier and the singlemode optical fiber;

FIG. 34 is the radio-frequency spectrum of the signal of thedual-wavelength pulse laser output, after being passed through theoptical amplifier and the single mode optical fiber to realize themulti-frequency optical comb, and then through the photodetector.

EXAMPLES

In the optical asynchronous sampling signal measurement apparatus, thepulsed light source contains only one optical resonant cavity, andoptical pulses with two repetition frequencies are produced by the sameresonant cavity. Because these two pulse sequences possess differentmodes, different polarization states, different central wavelengths orother different characteristics, using the modal dispersion, thepolarization mode dispersion, the birefringence or the chromaticdispersion of the related devices in the resonant cavity, one resonantcavity can emit optical pulse sequence with two different repetitionfrequencies. In the examples below, a pulsed light source can be amode-locked laser, based on the chromatic dispersion in the opticalcavity, it is realized that one pulsed light source emits pulsesequences with two different wavelengths and, thus, different repetitionfrequencies. In addition, a continuous-wave (CW)-laser-pumped microringresonator can also be used as the pulsed light source to produce anoptical frequency comb using the optical Kerr effect, by leveraging theslight difference of the refractive indices of the microring resonatorwhich can also be added into the optical resonant cavity, to achieve apulsed light source which emits optical pulse sequences with differentrepetition frequencies based on the birefringence dispersion.

The pulse light source used in the following examples from the first tothe seventh is a dual-wavelength mode-locked laser, which useserbium-doped fiber as the gain medium, and adjusts the intracavity gainspectrum by controlling the intracavity loss to realize the output ofthe dual-wavelength pulse laser at 1530 nm and 1560 nm Because of thechromatic dispersion of the fiber or other devices in the fiber cavity,the group velocities of the two wavelengths are different, and therepetition frequencies of the pulses at the two wavelengths aredifferent. Assuming the repetition frequency of the pulse sequence with1530 nm wavelength is f₁, and the repetition frequency of the pulsesequence with 1560 nm wavelength is f₂, the frequency difference is Δf.

Example 1

The optical asynchronous sampling signal measurement apparatus using thedual wavelength mode-locked laser for the terahertz signal measurementis shown in FIG. 2. Through a wavelength division multiplexer (WDM), thepulses with the center wavelength of 1530 nm and 1560 nm from thedual-wavelength mode-locked laser can be separated. The light pulse withthe center wavelength of 1560 nm can be passed through the optical poweramplifier to realize the power amplification and the pulse compression,and passed through a polarization control device to generate ahorizontal-polarized pump pulse sequence. The light pulse with thecenter wavelength of 1530 nm can be passed through the polarizationcontrol device to generate a 45-degree linear-polarized, reference pulsesequence.

The pump optical pulse sequence can be incident at an angle of 45 degreeupon the terahertz emission device—a piece of InAs crystal underexternal magnetic field based on the magnetic-field-enhanced Dembereffect, to radiate the terahertz signal in the direction of reflection.After being collimated by the first off-axis parabolic mirror, theterahertz beam transmits a distance through the device under test andthe air, and becomes the signal pulse sequence. Then after being focusedby the second off-axis parabolic mirror, the signal pulse sequence andthe reference pulse sequence are incident upon the terahertz receivingdevice—an electro-optic polymer film. After passing through theelectro-optic polymer film, the signal pulse sequence can be incidentonto the Wollaston prism and can be divided into two beams. These twolight beams can be directed onto two probes of a balanced photodetector,and the asynchronous sampling signal from the balanced photodetector canbe measured by an oscilloscope. The actual time step of the signal isΔf/f₂ times the original time step on the oscilloscope, and this yieldsthe terahertz time-domain signal, and via the Fourier transform, theterahertz time-domain spectroscopy information can be obtained.

Example 2

The optical asynchronous sampling signal measurement apparatus using thedual-wavelength mode-locked laser for the terahertz signal measurementis shown in FIG. 3. Through a wavelength division multiplexer, thepulses with the center wavelength of 1530 nm and 1560 nm from thedual-wavelength mode-locked laser can be separated. The light pulse withthe center wavelength of 1560 nm is passed through the polarizer togenerate a horizontal-polarized pump pulse sequence. The light pulsewith the center wavelength of 1530 nm is passed through the opticalpower amplifier and the standard single mode optical fiber with thenonlinear optical effect to realize the power amplification and pulsecompression, which is further incident on a frequency doubling BBOcrystal to generate the frequency doubling light at 765 nm as thereference pulse sequence.

The pump optical pulse sequence is incident at an angle of 45 degreeupon the terahertz emission device—the GaAs crystal, and radiates theterahertz wave by the optical rectification effect. After beingcollimated by the first off-axis parabolic mirror, the terahertz beamtransmits through the device under test and the air and becomes thesignal pulse sequence. Then after being focused by the second off-axisparabolic mirror, the signal pulse sequence and the reference pulsesequence are incident upon the terahertz receiving device—ZnTe crystal.Through the method of the electro-optic sampling we can detect theterahertz signal. After passing through the ZnTe crystal, the signalpulse sequence incident to a Wollaston prism can be divided into twobeams, and these two beams are incident to the two probes of a balancedphotoelectric detector, respectively, and the asynchronous samplingsignal from the balanced photodetector can be measured by anoscilloscope. The actual time step of the signal is Δf/f₂ times theoriginal time step on the oscilloscope, and this yields the terahertztime-domain signal, and through the Fourier transform, the terahertztime-domain spectroscopy information is obtained.

Example 3

The optical asynchronous sampling signal measurement apparatus using thedual-wavelength mode-locked laser for the terahertz signal measurementis shown in FIG. 3. Through a wavelength division multiplexer, thepulses with the center wavelength of 1530 nm and 1560 nm from thedual-wavelength mode-locked laser is separated. The light pulse with thecenter wavelength of 1560 nm is passed through the optical poweramplifier and the standard single mode optical fiber with the nonlinearoptical effect to realize the power amplification and pulse compression,and is further incident on a frequency doubling crystal BBO crystal togenerate the frequency doubling light at 780 nm as the pump light pulsesequence. The light pulse with the center wavelength of 1530 nm ispassed through the optical power amplifier and the standard single modeoptical fiber with the nonlinear optical effect to realize the poweramplification and pulse compression, and is further incident on afrequency doubling crystal BBO to generate the frequency doubling lightat 765 nm as the reference pulse sequence.

The pump optical pulse sequence is incident at the terahertz emissiondevice—the ZnTe photoconductivity switching and radiates the terahertzwave. After being collimated by the first off-axis parabolic mirror, theterahertz beam transmits through the test device and the air and becomesthe signal pulse sequence. Then after being focused by the secondoff-axis parabolic mirror, the signal pulse sequence and the referencepulse sequence are both incident upon the terahertz receiving device—theZnTe photoconductivity switching. Through the method of theelectro-optic photoconductivity sampling we can detect the terahertzsignal. After passing through the ZnTe photoconductivity, the signalpulse sequence incident to a Wollaston prism and can be divided into twobeams, and these two light beams are incident to the two probes of thebalanced photoelectric detector, respectively, and the asynchronoussampling signal from the balanced photodetector can be measured by anoscilloscope. The actual time step of the signal is Δf/f₂ times theoriginal time step on the oscilloscope, and this yield the terahertztime-domain signal, and through the Fourier transform, the terahertztime-domain spectroscopy information is obtained.

Example 4

The optical asynchronous sampling signal measurement apparatus using thedual-wavelength mode-locked laser for the pump-probe measurement isshown in FIG. 5. Through an optical filter, the pulses of differentrepeat frequencies with the center wavelength of 1530 nm and 1560 nmfrom the dual-wavelength mode-locked laser is separated. The light pulsewith the center wavelength of 1560 nm is passed through the opticalpower amplifier to realize the power amplification and pulse compressionand generate a reference pulse sequence. The light pulse with the centerwavelength of 1530 nm is passed through the power control device togenerate a signal pulse sequence. The signal pulse sequence and thereference pulse sequence can be merged into one beam by the fibercoupler, then incident on the device under test and output the lightsignal. The light signal is passed through the optical filter to get thesignal pulse sequence filtered, which is detected by the photodetectorto generate the asynchronous sampling signal. The asynchronous samplingsignal is measured by the oscilloscope and the actual time step of thesignal is Δf/f₁ times the original time step on the oscilloscope, thisyield pump probe signal.

Example 5

The optical asynchronous sampling signal measurement apparatus using thedual-wavelength mode-locked laser for the pump-probe measurement isshown in FIG. 6. The pulses of different repeat frequencies from thedual-wavelength mode-locked laser incident on the device under testtogether. The light pulse with the center wavelength of 1560 nm is thereference pulse sequence and the light pulse with the center wavelengthof 1530 nm is the signal pulse sequence. The light signals which passthrough the device under test input into the optical filter to filterout the signal pulse sequence, which is detected by the photodetectorand then measured by the oscilloscope to get the asynchronous samplingsignal. The actual time step of the signal is Δf/f₁ times the originaltime step on the oscilloscope and yield pump probe signal.

Example 6

The optical asynchronous sampling signal measurement apparatus using thedual wavelength mode-locked laser for the pump-probe measurement isshown in FIG. 7. Through an optical filter, the pulses of differentrepeat frequencies with the center wavelength of 1530 nm and 1560 nmfrom the dual-wavelength mode-locked laser is separated. The light pulsewith the center wavelength of 1560 nm is passed through the opticalpower amplifier to realize the power amplifier and pulse compression andgenerate a reference pulse sequence. The light pulse with the centerwavelength of 1530 nm is passed through the optical power amplifier torealize the power amplifier and pulse compression, and is furtherincident on a frequency doubling crystal BBO to generate the frequencydoubling light at 780 nm as the signal pulse sequence. The non-collinearsignal pulse sequence and reference pulse sequence focus on the deviceunder test through an optical lens, then the signal pulse sequence isdetected by the photodetector to get the asynchronous sampling signal.The actual time step of the signal is Δf/f₁ times the original time stepon the oscilloscope and yield pump probe signal.

Example 7

The optical asynchronous sampling signal measurement apparatus using thedual-wavelength mode-locked laser for the pump-probe signal measurementis shown in FIG. 7. Through an optical filter, the pulses of differentrepeat frequencies with the center wavelength of 1530 nm and 1560 nmfrom the dual-wavelength mode-locked laser is separated. The light pulsewith the center wavelength of 1530 nm is passed through the opticalpower amplifier to realize the power amplification and pulsecompression, and is further incident on a frequency doubling crystal BBOto generate the frequency doubling signal at 765 nm. The light is passedthrough a polarized beam splitter prism to be horizontal polarized andbecame a reference pulse sequence. The pulse light with the centerwavelength of 1560 nm is passed through the optical power amplifier torealize the power amplification and pulse compression, and is furtherincident on the frequency doubling crystal BBO to generate the 780 nmlight which is passed through a polarizer to be 45 degree linearpolarized and become a signal pulse sequence. The non collinear signalpulse sequence and reference pulse sequence are passed through the lensto focus on the device under test. The signal pulse sequence is passedthrough the analyzer whose polarization direction is vertical to thepolarizer and then measured by the oscilloscope. The actual time step ofthe signal is Δf/f₁ times the original time step on the oscilloscope andyield pump probe signal.

Example 8

The pulsed light source in this example is a dual-wavelength, dualfrequency pulse laser, which outputs two optical pulse sequences withdifferent repetition frequencies, where the frequency difference is 472Hz, and the center wavelengths are 1532 nm and 1555 nm respectively.Through an optical filter, the pulses of different repeat frequencieswith the center wavelength of 1532 nm and 1555 nm from thedual-wavelength mode-locked laser is separated. The optical pulsesequence with the center wavelength of 1555 nm is the first opticalpulse sequence, which is passed through the device under test togenerate a signal pulse sequence. The optical pulse sequence with thecenter wavelength of 1532 nm is the second optical pulse sequence, whichis passed through the optical amplifier and the standard single-modefiber to realize the spectral broadening and become the reference pulsesequence. Its spectrum is overlapped with the spectrum of signal pulsesequence. The signal pulse sequence and the reference pulse sequencemerge in the coupler, and then detected by the photodetector to outputthe electric signal in the time domain. The spectroscopy information canbe obtained after the time axis transform and the time to frequencydomain transform. In this example, either optical pulse sequence canhave the spectrum overlapped with another through the spectrumbroadening.

Example 9

The pulsed light source used in this example is a dual-wavelengthmode-locked laser. The principle of dual wavelength output is tuning theshape of the gain spectrum of the erbium doped fiber through controllingthe intracavity loss, so that the gain at different wavelength is thesame and to realize dual wavelength mode-locked pulse.

The structure of the laser is shown in FIG. 10, which is a passivelymode locked fiber laser with the ring cavity structure. The pump lightsource is a semiconductor laser 1003 of 1480 nm wavelength. The pumplight is coupled into the erbium doped fiber (EDF) 1001 through the1480/1550 wavelength division multiplexer 1002. The EDF is 5 meters longand its absorption coefficient at 1530 nm is 6.1 dB/m. The EDF isconnected with the optical isolator 1008 to ensure the unidirectionaltransmission of the light in the fiber cavity. The polarizationcontroller 1006 in the cavity is used to control the polarization state.The mode locked device is the carbon nanotube/polyimide film 1003 andthe thickness of the film is 45 microns. The loss of the mode lockeddevice is about 4 dB when the film is sandwiched between two FC/PCconnectors.

In order to ensure the anomalous average dispersion in the cavity togenerate soliton pulse, a 6.85 m standard single-mode optical fiber 1004(including all pigtailed devices) is additionally added into the cavityand the total length of the single-mode optical fiber is 11.85 m in thecavity. The 80/20 optical coupler 1007 outputs the 20% laser light tothe outside of the cavity, and returns 80% back. The intracavity lossmakes the gain of the EDF in the vicinity of 1530 nm and 1560 nm be thesame to meet the condition of generating dual wavelength mode locking.

When the pump power is about 80 mW, by introducing a vibrationperturbation in the optical cavity, we can achieve the dual wavelengthmode locking, and the center wavelength is 1532.46 nm and 1547.43 nm, asshown in FIG. 11. The spectrum of the output pulse is detected by a fastphotoelectric detector and a spectrum analyzer, as shown in FIG. 12.Because of the chromatic dispersion in the optical cavity, the groupvelocity of the two wavelengths is different, so the repetitionfrequency of the two wavelength pulse is also different. As shown in theradio frequency spectrum diagram, the repetition frequency f₁ of thepulse with the center wavelength of 1532.46 nm is 34.518773 MHz. Therepetition frequency f₂ of the pulse with the center wavelength of1547.43 nm is 34.518156 MHz. The frequency difference Δf is 617 Hz, andthe pulses of the two wavelengths are oscillating at twice of thefundamental round-trip frequency of the cavity.

The optical path measurement apparatus of using the dual-wavelengthmode-locked laser is shown in FIG. 13. The dual-wavelength mode-lockedlaser 1301 emits the light pulses, which are passed through the opticalamplifier 1302 to realize the power amplification, and then the lightpulses enter a four-channel band-pass optical filter 1303, where thefilter channel with a passband of 1528.5 nm to 1536.5 nm can selectivelypass the light pulse with the center wavelength of 1532.46 nm. Theoutput spectrum is shown in FIG. 14. The filter channel with a passbandof 1546 nm to 1554 nm lets the light pulse with the center wavelength1547.43 nm to go through, and its output spectrum is shown in FIG. 15.

The light pulse with the center wavelength of 1547.43 nm is passedthrough the optical amplifier 1304 to realize the power amplification,and by using the nonlinear effect of the erbium doped fiber in theoptical amplifier and the single-mode fiber 1314 to generate thespectral broadening, as shown in FIG. 16. As seen from FIG. 16, thespectrum has been significantly broadened, and there is considerablespectral components near 1532 nm, i.e. it now has significant spectraloverlap with the light pulse with the center wavelength of 1532.46 nm,After passing through a bandpass filter 1306 with the passband of 1528nm-1536 nm, as the reference optical pulse sequence for the light pathmeasurement, its spectrum is shown in the spectrum in FIG. 17, where itspower is about 60 W.

The light pulse with the center wavelength of 1532.46 nm is amplified bythe optical amplifier 1305, and the power reaches about 15 mW, and thenthe pulse is input into the port of the circulator 131, and output fromthe port 132 as the first probe optical pulse sequence, which islaunched through a cleaved tip of single-mode optical fiber and passedthrough the lens 1307 with a focal length of 12 mm to become acollimated beam. Part of the beam is reflected off mirror 1308, whileanother part of the beam is reflected off a distant mirror 1309, andcoupled back into the optical fiber, and the light this time exitsthrough port 133 of the circulator.

The distance between the single mode optical fiber cleaved tip and themirror 1308 is about 18.5 cm, while the distance between the two mirrorsis about 29 cm. The optical path difference between the two mirrors willgenerate a relative delay ti between the pulses reflected from themrespectively. The second probe pulse sequence output from the circulatorport 133 and the reference optical pulse sequence are passed through thepolarization controller 1309 and 1310, respectively, and are input intothe 50/50 3 dB coupler 1311, which combines them, and then the lightfrom the two output ports of the coupler is incident onto the two probesof the balance detector 1312. The output signal of the balance detector1312 is detected by the oscilloscope 1313, and the time domaincorrelation signal can be obtained as shown in FIG. 18. It can be seenfrom FIG. 18, that there are 3 correlation peak signals, which areresultant from the light reflected back by the single mode fiber cleavedtip, mirror 1, and mirror 2 interfering with the reference optical pulsesequence, where the measured time differences Δτ between three peaks are70 us and 109 μs, respectively. Based on these numbers, and according tothe equations of the time difference between the pulses τ=Δτ×Δf/f_(p)and the optical path difference d=v_(g)*τ, the optical path between thesingle-mode fiber cleaved tip and the mirror 1 is determined as 37.5368cm, and the optical path between the mirror 1 and the mirror 2 is58.8262 cm.

In the above apparatus of determining the optical distance informationthrough measuring the optical field correlation signal, nonlinearoptical devices exists at least in either the signal light path or thereference light path, in order to broaden the spectrum of the lightthrough that path. The effect of the optical amplifier is to amplify theoptical signal, which enables that the amplified light can generatesufficiently strong nonlinear effects through passing the nonlinearoptical devices, so that the spectrum after being broadened or shiftedcan overlap with the spectrum of the light in the other path. If thelight before any amplification is strong enough to generate such anoverlap, the use of the optical amplifier may not be necessary. Thefunction of the polarization control device is to adjust thepolarization state of the light signal, so that the two signals satisfythe polarization relationship required by the field correlation orintensity correlation measurement. If the two signals can generate thecorrelation signal before the adjustments, the use of the polarizationcontroller is no longer necessary. The optical filter 1306 is to ensurethat the spectra of the reference pulse sequence and the signal pulsesequence have similar center wavelengths, and is also not necessary.

Example 10

The principle of the dual-wavelength mode-locked laser used in thisexample is the same as the laser used in Example 9, where the opticalpulses output by the dual-wavelength mode-locked laser pass through theoptical splitting device, which divides the optical pulse with thecenter wavelength of 1532.46 nm and the optical pulse with the centerwavelength of 1547.43 nm into two independent branches. One of the twooptical pulses is used as a reference optical pulse sequence with itspulse width of 0.6 ps and another optical pulse is used as the firstoptical pulse sequence with its pulse width of 1 ps. The first opticalpulse sequence merges to the signal optical pulse sequence after it goesthrough two different optical delays in the way of transmission. Afteradjusting the polarization state, the parallel beam of the signaloptical pulse sequence and the reference optical pulse sequence arefocused by a lens onto the second order nonlinear optical material, suchas the frequency doubling crystal, BBO. The photomultiplier tube isplaced after the BBO and collects the intensity correlation signal. Theintensity correlation signal curve similar to that shown in FIG. 18 canbe measured, where the intensity correlation signal of each peak isshown in FIG. 19. According to the measured time difference of thecorrelation signal in the sequence, the optical path information can beobtained by using the method similar to Example 9.

In the above apparatus, which get the optical path information bymeasuring the intensity related signal, the optical amplifier is used toamplify the optical signal, so that it can generate the intensitycorrelation signal through the nonlinear photoelectric detector. If theoptical signal can generate the intensity correlation signal beforeamplification, then the optical amplifier is not necessary. Thepolarization controller device adjusts the polarization state of theoptical signal to satisfy the demands of polarization relationship inintensity correlation. If the two optical signals can generate thecorrelation signal before the adjustment, the polarization controller isnot necessary.

Example 11

FIG. 20 is a schematic diagram of the optical asynchronous samplingmeasurement apparatus. The dual frequency pulse laser 2001 outputs twooptical pulse sequences with different repetition frequencies. Thefrequency difference is 472 Hz, where the center wavelengths are 1532 nmand 1555 nm respectively. The output spectrum of the dual frequencypulse laser is shown in FIG. 21 and the radio frequency spectrum isshown in FIG. 22. The two optical pulse sequences are divided into twobranches through the optical splitting device, which can be band-passfilter 2002. The optical pulse with its center wavelength of 1532 nm canbe the first optical pulse sequence. The optical pulse with its centerwavelength of 1555 nm can be the second optical pulse sequence.

The first optical pulse sequence is divided into two branches afterpassing through the optical coupler 2003, where one branch becomes acalibration pulse sequence after the calibration delay and the otherbranch becomes a target pulse sequence after the target delay. Thecalibration pulse sequence and the target pulse sequence are passedthrough the optical coupler 2004 and merge into a signal pulse sequence.The second optical pulse sequence is amplified by the optical amplifier2005 to generate a reference pulse sequence, which is passed through theoptical coupler 2006 to generate one optical beam and input into asemiconductor optical amplifier (SOA) 2007 as the pulse interactiondevice. The output optical pulse sequence of the SOA is filtered throughthe filter 2008 and interacts with the reference pulse sequence, whichis then converted to an electrical signal by the photoelectric detector2009, and finally the oscilloscope 2010 measures the electrical signal,as the time domain graph shown in FIG. 23. Because the SOA has thecharacteristic of gain saturation, when the reference optical pulse andthe signal optical pulse are coincident in the time domain, thereference optical pulse makes the SOA saturated and the transmittance ofthe signal optical pulse decreases. By measuring the time difference ofthe two falling edges Δτ, the distance difference between the targetdelay and the calibration delay can be calculated from d=v_(g)ΔtΔf/f₁,where v_(g) is the group velocity of the signal pulse sequence.

Example 12

FIG. 24 is another schematic diagram of the optical asynchronoussampling measurement apparatus. The difference between this example andthat shown in Example 11 is using the all-optical switch as a pulseinteraction device. The dual frequency pulse laser 2401 outputs twooptical pulse sequences with different repetition frequencies, where thefrequency difference is 472 Hz, the center wavelengths are 1532 nm and1555 nm, respectively. The optical pulse sequences are divided into twobranches through the optical splitting device, which can be a band-passfilter 2402. The optical pulse sequence with its center wavelength of1532 nm can be the first optical pulse sequence. The optical pulsesequence with its center wavelength of 1555 nm can be the second opticalpulse sequence. The first optical pulse sequence is passed through theoptical coupler 2403 and divided into two branches. One branch becomesthe calibration pulse sequence after the calibration delay. The otherbranch becomes a target pulse sequence after the target delay. Thecalibration pulse sequence and the target pulse sequence are passedthrough the optical coupler 2404 and merge into the signal pulsesequence. The second optical pulse sequence is amplified by the opticalamplifier 2405 to become the reference pulse sequence and controls theall-optical switch 2406 as the pulse interaction device pass light ornot. When the pulse of the reference pulse sequence and the signal pulsesequence are coincident in time, the pulse of the signal pulse sequencecan pass through the all-optical switching, otherwise it could not passthrough the all-optical switching. The optical pulse sequence output bythe all-optical switching is converted to an electrical signal by thephotodetector 2407 and received by the oscilloscope 2408. By measuringthe time difference of the two adjacent pulses Δτ in the electricalsignal, the distance difference between the target delay and thecalibration delay can be calculated from d=v_(g)ΔtΔf/f₁, wherein v_(g)is the group velocity of the signal pulse sequence.

Example 13

The structure of the dual-wavelength mode-locked laser used is shown inFIG. 25. The laser is a passively mode-locked fiber laser with ringcavity structure. The pump light source is a semiconductor laser 2503with its wavelength of 1480 nm, the pump light emitted is coupled intothe erbium doped fiber (EDF) 2501 through the 1480/1550 wavelengthdivision multiplexer 2502. The EDF is 5-meters-long and its absorptioncoefficient at 1530 nm is 6.1 dB/m. The EDF is connected to the opticalisolator 2508 to ensure the light unidirectional transmission in thefiber cavity. The polarization controller 2506 in the cavity is tocontrol the polarization state. The mode locker is the carbonnanotube/polyimide film 2503 and the thickness of the film is 45microns, the loss of the mode locker is 3.5 dB when the film issandwiched between two FC/PC connectors. The total length of thestandard single-mode optical fiber 2504 is 6.1 m in the cavity. The80/20 optical coupler 2507 outputs 20% of the light to the outside ofthe cavity, and returns 80% back.

The intracavity loss makes the EDF have two gain peaks around 1530 nmand 1560 nm and meets the demands for generating dual-wavelength modelocking. The center wavelength of the dual wavelength mode locking is1535 nm and 1557 nm, respectively. The output spectrum of the dualwavelength mode-locked laser is shown in FIG. 27. The radio-frequencyspectrum of the output pulse is detected by a fast photoelectricdetector and a spectrum analyzer, as shown in FIG. 28. Due to thedispersion of the optical fiber and other devices in the optical cavity,the group velocity of the two wavelengths is different, so that therepetition frequency of the optical pulse sequence with the twodifferent wavelengths (i.e. frequency interval of the optical comb) isalso different. As can be seen from the spectra, the repetitionfrequency ft of the pulse with the center wavelength of 1535 nm is14.489145 MHz, and the repetition frequency of the pulse with the centerwavelength of 1557 nm f₂ is 14.488649 MHz, where the frequencydifference is 496 Hz.

The output light of the dual wavelength laser 2601 is amplified by theoptical amplifier 2602, and then filtered by the optical filter 2603, tooutput two optical pulse sequences at two output ports. The opticalspectrums are shown in FIG. 29 and FIG. 30. The spectrum of the opticalpulse of 1557 nm is broadened by third order nonlinear effect (selfphase modulation, the four wave mixing, etc.) of the optical fiberamplifier 2604 and the single-mode fiber 2605, as shown in FIG. 31.After being filtered by the filter 2606, the spectrum around 1535 nm isshown in FIG. 32. The apparatus outputs the optical comb with differentfrequency interval in the wavelength range of 1535 nm. The opticalamplifier in the apparatus amplifies the optical signal, so that it canmake the spectrum broaden and overlap with the spectrum of anothersignal by the nonlinear effects. However, the optical amplifier is notnecessary, especially when the spectra of the optical signals areoverlapped before being amplified.

Example 14

The dual wavelength mode-locked laser used in this example is the sameas in Example 1. The output light is directly passed through theamplifier and the single-mode transmission optical fiber to broaden thespectrum of the light pulse sequence with different center wavelengthsand make their spectra overlap by using third-order nonlinear effect ofthe gain fiber and the single mode optical fiber (self phase modulation,the four wave mixing, etc.), so that there are two optical combs withdifferent frequency interval in the overlapping wavelength region, asshown in FIG. 33. The radio frequency spectrum of the signal after thephotoelectric conversion is shown in FIG. 34. It shows the apparatusrealizes the output of the optical comb with different frequencies. Theoptical amplifier in the apparatus is used to amplify the opticalsignal, so that it can make the spectrum broaden and overlap by thenonlinear effects. However, the optical amplifier is not necessary,especially when the spectrum of the optical signal can overlap beforebeing amplified.

The above are only exemplary embodiments of the present application, andshould not limit the breadth and the scope of protection. Manymodifications and variations will be apparent to one of ordinary skillin the art. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,thereby enabling a person having ordinary skill in the art to understandtechnology scope disclosed by the application.

What is claimed is: 1-39. (canceled)
 40. A method for measuring anoptical asynchronous sampling signal comprising the steps of: a)generating a first optical pulse sequence with a first repetitionfrequency (f1) from a pulsed light source; b) generating at least asecond optical pulse sequence with a second repetition frequency (f2)from the pulsed light source, where f1 is different from f2, and where adifference in repetition frequency (Δf) between f1 and f2 is equal tothe absolute value of f1 less f2; c) passing the first optical pulsesequence through a signal optical path to transform the first opticalpulse sequence into a signal pulse sequence; d) passing the secondoptical pulse sequence through a reference optical path to transform thesecond optical pulse sequence into a reference pulse sequence; and e)passing the signal pulse sequence and the reference pulse sequence intoa detection device to generate an asynchronous sampling signaldetermined by f1 and f2.
 41. The method of claim 40, further comprisingthe step of performing a time-axis transformation on the asynchronoussampling signal to obtain a transformed asynchronous sampling signalusing transform formula ΔT=τΔf/f1, wherein Δτ is temporal position ofthe asynchronous sampling signal, ΔT is actual temporal positiondetermined after the time-axis transformation, and temporal informationcan be obtained based on the transformed asynchronous sampling signal,and spectral information can be further obtained by means oftransforming between a time domain and a frequency domain.
 42. Themethod of claim 40, further comprising inputting the first optical pulsesequence into a terahertz emission device, where a terahertz pulsesequence is generated, and the signal pulse sequence is formed.
 43. Themethod of claim 42, further comprising passing the terahertz pulsesequence through a device under test to generate the signal pulsesequence.
 44. The method of claim 42, further comprising inputting thefirst optical pulse sequence through one or more devices selected fromthe group consisting of power control devices, pulse waveformtransformers, polarization controllers and nonlinear optical devicesbefore entering a terahertz emission device.
 45. The method of claim 42,where the second optical pulse sequence is transformed into thereference pulse sequence through one or more devices selected from thegroup consisting of power control devices, pulse waveform conversiondevices, polarization controllers and nonlinear optical devices.
 46. Themethod of claim 42, where the signal pulse sequence and the referencepulse sequence are input into a terahertz receiving device as thedetection device to obtain the asynchronous sampling signal.
 47. Themethod of claim 42, further comprising processing the asynchronoussampling signal to yield terahertz temporal waveform information andterahertz time-domain spectroscopy information.
 48. The method of claim42, further comprising inputting the signal pulse sequence and thereference pulse sequence into the device under test, and then measuringcharacteristics of the signal pulse sequence using an optical detectoras the detection device to obtain the asynchronous sampling signal. 49.The method of claim 48, where the first optical pulse sequence istransformed into the signal pulse sequence after passing through one ormore devices selected from the group consisting of power controldevices, pulse waveform transformers, polarization controllers andnonlinear optical devices.
 50. The method of claim 48, where the secondoptical pulse sequence is transformed into the reference pulse sequencethrough one or more devices selected from the group consisting of powercontrol devices, pulse waveform transformers, polarization controllersand nonlinear optical devices.
 51. The method of claim 48, where afterprocessing of the asynchronous sampling signal, the pump-probeinformation of the device under test can be obtained.
 52. The method ofclaim 42, further comprising inputting the signal pulse sequence intothe device under test, and the signal pulse sequence and the referencepulse sequence interact in the detection device to generate theasynchronous sampling signal.
 53. The method of claim 52, where thefirst optical pulse sequence input into a device under test to generatesignal pulse sequence after it passing through one or more devicesselected from the group consisting of power control devices, pulsewaveform transformer, polarization controllers and nonlinear opticaldevices.
 54. The method of claim 52, where the second optical pulsesequence is transformed into the reference pulse sequence through one ormore devices selected from the group consisting of power controldevices, pulse waveform transformer, polarization controllers andnonlinear optical devices.
 55. The method of claim 52, where theasynchronous sampling signal is processed to obtain time-domainspectroscopy information of the device under test.
 56. A method formeasuring an optical distance difference between a target optical pathand a calibration optical path, comprising the steps of: a) generating afirst optical pulse sequence and a second optical pulse sequence from apulsed light source; b) splitting the first optical pulse sequence intoa first branch and a second branch; c) passing the first branch throughthe calibration optical path to transform the first branch into acalibration pulse sequence; d) passing the second branch through thetarget optical path to transform the second branch into a target pulsesequence; e) merging the calibration pulse sequence and the target pulsesequence to generate the signal pulse sequence; f) passing the secondoptical pulse sequence through a reference optical path to transform thesecond optical pulse sequence into a reference pulse sequence; and f)passing the signal pulse sequence and the reference pulse sequence intoa detection device to generate an asynchronous sampling signal, wherethe time difference between the target pulse sequence and thecalibration pulse sequence in the signal pulse sequence are calculatedfrom the asynchronous sampling signal and the time difference is furtherused to measure the optical distance difference between the targetoptical path and the calibration optical path.
 57. The method of claim56, where one or both the first optical pulse sequence and the secondoptical pulse sequence pass through one or more devices selected fromthe group consisting of a power control device, a pulse waveformtransformer, a polarization controller and a nonlinear optical device.58. The method of claim 56, where the reference pulse sequence and thesignal pulse sequence interact in the detection device to generate oneor both a field-dependent and an intensity-dependent signal as theasynchronous sampling signal.
 59. The method of claim 56, where thereference pulse sequence and the signal pulse sequence interact in apulse interaction device, where pulses of the signal pulse sequence thatare overlapped with pulses of the reference pulse sequence in a temporaldomain are changed after the interaction, where changes in pulses of thesignal pulse sequence are measured to yield the asynchronous samplingsignal.